Processing of syntax in music: an EEG study

Abstract

In order to make sense out of a sequence of sounds in music, our brain must meaningfully fit and recombine acoustic events into a hierarchic online stream. Although these information units are auditively delivered in sequences with local connections (one after the other), it is assumed that long-term dependencies are established counting on memory traces to sustain recursiveness in time. Despite theoretical and empirical consensus, there is yet no clear physiological evidence of the temporal dimension of syntactic relations in music. We investigated whether there is quantifiable neural activity suggesting the existence of a mental representation for fundamental music syntax rules like tonic-dominant-tonic chords. For such, we compared brain electric activity in 24 subjects (12 musicians, 12 non-musicians) aroused by original and harmonically modified versions of J.S. Bach chorales, using electroencephalography (EEG). Chorales were built by two phrases: initiated by a tonic and arriving in a dominant chord two bars away (first phrase), and concluded in a tonic chord three bars after the dominant (second phrase). Modified versions were created either by elevating or lowering the first, therefore keeping the second phrase intact. We compared the brain electric response for the last chord in both versions. Our data produced event related potentials (ERP) which suggest that the subjects’ brain processed modified versions differently than originals. We observed an amplitude difference in the negativities peaking around 210 ms after the last chord onset. This finding replicates previous studies on harmonic disruptions processed in the same latency, but differs regarding brain source contributions: our data revealed a more posterior than anterior effect. Considering that last chords were the same acoustic event in both versions, we hypothesize that the amplitude differences plausibly indicate that the syntactic expectancy (long-term dependency between the tonic, dominant and tonic chord) may relate to brain mechanisms of processing non local connections, establishing a hierarchical storage of acoustic events in memory. We interpret that our parietal findings parallels with literature on mathematical sequence processing which suggest posterior brain regions to be engaged on complex calculation requiring the storage of numbers magnitude in memory for further computation. Our study overcame an obstacle not to date surpassed: observe and separate brain electric activity raised by long-term syntactic disruption in music from the overlap of local mismatch detection.